In general, the present invention relates to the fabrication of tiny conductive devices sized on the order of the microcircuits, and even smaller, that operate as active elements on printed circuit boards, or any other such support structure, of varying sizes (including microchip-sized to the level of so-called molecular electronics where one or a small collection of molecules is capable of operating as an active electronic element). The fabrication of extremely small reliable components and complex circuits, although difficult, is very important to the ongoing development and distribution of miniaturized computerized contraptions ranging from analytical instruments and testing equipment (whether simple or complex) such as sensors, voltmeters, data collection equipment, and so on, to consumer devices such as notebook computers, multifunctional palm-sized computers, watch-sized computerized cellular communication devices, etc.
More particularly, the invention relates to a new apparatus that incorporates a unique method for dispersing a plurality of elongated nano-sized elements within a carrier-fluid to assemble any of a number of different tiny conductive devices built to replace a wide variety of conventional devices or built to the specifications of new conductive devices; such devices to include: diodes (used as light emitters and sensors, switches, etc.), transistors, on-tube junctions, capacitors, inductors, resistors, oscillators, MEMS (MicroElectroMechanical System) technology elements/devicesxe2x80x94tiny mirrors, sensors, light reflectors, switches, microactuators, read/write heads, etc.
The apparatus includes a nozzle or orifice through which the elongated nano-elements within the carrier-fluid are directed such that they pass through an electromagnetic (EM) field and toward a first charge-receptive area of a support surface. This charge-receptive area has been given a charge to attract at least an end-portion of one of the nano-elements. The amount of charge held by the charge-receptive area being targeted depends upon the charge imparted to one or both ends of the nano-element upon passing through the EM field. A second charge-receptive area having a charge to attract either a second end-portion of the first nano-element or an end-portion of a second nano-element is preferably included; this second area in proximity to the first charge-receptive area, multiple charge-receptive areas may be patterned as needed.
If the carrier-fluid is in liquid form, the liquid is chosen so that preferably a substantial amount of it evaporates once the nano-element has attached to the charge-receptive area being targeted. If the carrier-fluid is in gas form, to minimize unwanted turbulence of the gas being discharged from the nozzle in an effort to better control flow and direction of nano-elements toward, as well as attachment thereof to a respective the charge-receptive area, preferably the area around the nozzle and charge-receptive area being targeted is under vacuum (i.e., the pressure around the charge-receptive area being targeted is less than the surrounding area) or the nano-elements within carrier-fluid are dispersed and directed toward target charge-receptive areas under high pressure.
Carbon nanotubes belong to a small family of carbon compounds known as fullerenes. These tube-like structures may have single walls or multiple walls, generally each nanotube wall is essentially one carbon atom thick. They have been described as tiny strips of graphite sheet rolled into tubes and capped with half a fullerene at each end. Nanotubes may be multiwalled (made up of several concentric hollow cylinders of carbon atoms nested inside each other) or single-walled with an outer diameter on the order of 1 nanometer (a billionth of a meter) and length varies from several microns to 100+ microns depending upon, among other things, fabrication method used to form the nanotubes. Nanotubes are very strong, stable (chemically inert), lightweight, and can withstand repeated bending, buckling, and twisting-plus they are efficient heat transfer agents. Atoms in a nanotube arrange themselves in hexagonal rings like chicken wire.
Graphite is a semimetal: Whereas most other electrical conductors can be classified as either a metal or a semiconductor, graphite is balanced in the transitional zone between the two. This is due to the unique properties of the building material of graphite, namely, carbon. Under intense pressure, carbon atoms form bonds with four neighboring carbons, creating the pyramidal arrangement of diamond. When carbon forgoes that fourth bond and links up with only three neighbors, it creates the hexagonal rings in graphite""s structure. This arrangement leaves graphite with a host of unpaired electrons, which effectively xe2x80x98floatxe2x80x99 above or below the plane of carbon rings. These floating electrons are more or less free to buzz around graphite""s surface, which makes it a good electrical conductor. It is, however, these unattached bonds that leaves carbon atoms at the border of a graphite sheet susceptible to reaction with something nearby. This characteristic is what allows a heated (1200 degrees Celsius, or so) graphite sheet to curl back against itself, inter-knit together, and form a tiny cylindrical graphite element now commonly referred to as a nanotube.
In a graphite sheet, one particular electron state (called the Fermi Point) gives graphite almost all of its conductivity; none of the electrons in other states are free to move about. Statistically, only a fraction (estimated at one-third) of graphite walled nanotubes of any collection will act as truly metallic nanowires, while the remaining two-thirds will operate like semiconductors. Meaning that these nanotubes do not conduct current easily without an additional boost of energy (by way of a burst of light or sufficient voltage) to knock electrons from valence states into conducting states along the nanotube. The amount of energy needed depends on the separation between the two levels and is the so-called band gap of a semiconductor. It is this band gap that makes semiconductors useful in circuits. Carbon nanotubes do not all have the same band gap, because for every circumference there is a unique set of allowed valences and conduction states. The smaller-diameter nanotubes have very few states that are spaced far apart in energy. As the diameter increases, more and more states are allowed and the spacing between them shrinks. In this way, different-sized nanotubes can have band gaps as low as zero (similar to a metal), or as high as the band gap of silicon, and almost anywhere in between-making it readily tuned. It is predicted that multiwalled nanotubes have even more complex behavior, as each layer in the tube has its own, individual geometry.
In connection with describing a computer-designed model of nanotube gears that have benzene groups arrayed around the nanotube to act as cogs whereby, as a nanocylinder rolls, its tiny teeth turn the nanotube like a microscopic drive shaft, SCIENCE magazine author Robert F. Service admits that fabrication remains an issue (xe2x80x9cSuperstrong Nanotubes Show They Are Smart, Tooxe2x80x9d, Aug. 14, 1998, Vol. 281, see pg. 942): xe2x80x9c[n]anogears are likely to remain simulations for some time, however, as there""s no obvious way to build them.xe2x80x9d As reported in the December 2000 issue of Scientific American (xe2x80x9cNanotubes for Electronicsxe2x80x9d, Philip Collins and Phaedon Avouris, see pg. 66) the authors explain their labor-intensive method of forming a FET: xe2x80x9cWe should emphasize, however, that so far our circuits have all been made one at a time and with great effort. The exact recipe for attaching a nanotube to metal electrodes varies among different research groups, but it requires combining traditional lithography for the electrodes and higher-resolution tools such as atomic force microscopes to locate and even position the nanotubes.xe2x80x9d Thus, current fabrication methods fall short of being feasible in large-scale nano-size device production. As further reported by Collins and Avouris, when oriented on-end and electrified, carbon nanotubes will act like a lightning rod, concentrating the electrical field at its tip. Because the ends are so xe2x80x98sharpxe2x80x99, such charged nanotubes efficiently emit electrons at lower voltages (a behavior called xe2x80x9cfield emissionxe2x80x9d) than electrodes made from most other materials. The fabrication and properties of nanotubes as well as speculation on potential uses and applications, have been the subject of numerous publications.
Though the apparatus and method of assembling a conductive device of the invention provides a means by which batch processing of nano-sized devices and elements may be more cost-effectively achieved utilizing the relatively recently identified elongated structures referred to as nanotubes, other elongated conductive elements (whether filled-in in a nanorod shape or hollow as are nanotubes) with similar aspect ratios having sufficient structural integrity to withstand dispersion within a suitably selected carrier-fluid through a nozzle, have been developed and are referred to in more detail in the following technical papers, each of which is incorporated herein by reference to the extent details of elongated structures for use according to the invention, are set forth:
(A) E. W. Wong, P. E. Sheehan and C. M. Lieber, xe2x80x9cNanobeam Mechanics: Elasticity, Strength and Toughness of Nanorods and Nanotubesxe2x80x9d, Science, 277, 1971-1975 (1997);
(B) H. Dai, E. W. Wong, Y. Z. Lu, S. Fan, and C. M. Lieber, xe2x80x9cSynthesis and Characterization of Carbide Nanorods,xe2x80x9d Nature, 375, 769 (1966);
(C) E. W. Wong, B. W. Maynor, L. D. Burns and C. M. Lieber, xe2x80x9cGrowth of Metal Carbide Nanotubes and Nanorodsxe2x80x9d, Chem. Mater. 8, 2041-2046 (1996);
(D) Mona B. Mohamed, Kamal Z. Ismail, Stephan Link, and Mostafa A. El-Sayed, xe2x80x9cThermal Reshaping of Gold Nanorods in Micellesxe2x80x9d, JPhysChemB, 102 (47), 9370-9374 (1998);
(E) B. R. Martin, D. J. Dermody, B. D. Reiss, M. Fang, L. A. Lyon, M. J. Natan, and T. E. Mallouk, xe2x80x9cOrthogonal Self Assembly on Colloidal Gold-Platinum Nanorods,xe2x80x9d Adv. Mater., 11, 1021-1025 (1999); and
(F) S. Link, M. B. Mohamed, and M. A. El-Sayed, xe2x80x9cSimulation of the Optical Absorption Spectra of Gold Nanorods as a Function of Their Aspect Ratio and the Effect of the Medium Dielectric Constantxe2x80x9d, JPhysChemB, 103 (16), 3073-3077 (1999).
Microelectronics is that area of electronics technology associated with the fabrication of electronic systems or subsystems using extremely small (microcircuit) components. The conventional method by which microelements and microprocessors are fabricated is by way of a series of layering steps are performed. Microcircuit wafer fabrication generally starts with a substrate to which layers, films, and coatings (such as photoresist) can be added or created (e.g., when fabricating a MOS monolithic IC, a silicon oxide layer is created on top of the silicon wafer), and from which these added or created materials can be subtractively etched (e.g., as in dry etching). Additionally, throughout semiconductor wafer fab, various processes are used to clean wafers so that surfaces are reproducible and stable (see for general reference, xe2x80x9cMicroelectronics: Processing and Device Designxe2x80x9d by Prof. Roy A. Colclaser, John Wiley and Sons (1980), pg. 82). The substrate for a microelectronic circuit is the base upon which the circuit is fabricated. The use of silicon and its oxide, along with photolithography, in semiconductor wafer fabrication dates back to the 1950""s. A substrate must have sufficient mechanical strength to support its circuit(s) during fabrication, and substrate electrical characteristics depend on the type of microcircuit being fabricated.
It is a primary object of this invention to provide an apparatus for dispersing a plurality of conductive elongated nano-elements distributed within a carrier-fluid (whether in liquid or gas state) toward at least one charge-receptive area of a support surface to assemble a conductive device therewith. It is a further object to provide a conductive device for use and operation as an electrically-driven device or as a component of an electrical device, comprising a first charge-receptive area of a support surface to which at least an end-portion of one of a plurality of elongated nano-elements has attached. The charge-receptive area has, at least initially upon dispersion of the carrier-fluid with nano-elements, a charge that attracts the nano-element end-portion that ultimately attaches. It is also an object of this invention to provide a method of assembling such a conductive device having a first charge-receptive area of a support surface to which at least one of a plurality of elongated nano-elements has attached. The method comprises the steps of: charging the first charge-receptive area to attract at least one of the nano-elements in the plurality; dispersing from a nozzle, the plurality of elongated nano-elements distributed within a carrier-fluid such that the nano-elements pass through an electromagnetic (EM) field and toward at least the first charge-receptive area.
The innovative apparatus, method, and conductive devices produced therewith, as contemplated and described herein, has been designed to accommodate a variety of alternatives, including but not limited to the following list of features: dispersing nano-elements within a carrier-fluid that is in a liquid or gas state, with the apparatus configured accordingly to have a micro-sized nozzle suitable for directing the carrier-fluid containing the nano-elements out of an associated carrier-fluid reservoir, suitable techniques/technologies for charging or creating the charge-receptive area(s) or patterns such that respective nano-elements are attracted thereto, including direct application of ions to the substrate surface in a manner similar to any of those used to create the charged or electrostatic latent image used in ionography printing, or utilizing techniques applied in the formation of an electrostatic latent image as is done in xerography printing, and so on; many different types of substrate supports made of a wide variety of materials are contemplated; many different shapes of conductive nano-elements including nanorods and the class of graphite elongated hollow elements commonly referred to as nanotubes; many different configurations/patterns of charge-receptive areas on the various support surfaces are contemplated as are a wide variety of conductive device structures made according to the invention, whether single- or multiwalled/layered structures built atop one or more charge-receptive areas of the substrate surface and/or projections or protrusions therefrom.
Furthermore, in the spirit of design goals contemplated by this disclosure, whether expressed: (a) The simple, innovative apparatus and method may be carried out by employing, or readily tailored to use, currently-available subassemblies and computer processors, as well as associated storage and memory, etc., to control the quality and quantity of assembling conductive devices of the invention in small batches or in large numbers in a fully- or partially-automated assembly-line fashion; and (b) In connection with efforts to miniaturize electrical equipment and systems, conductive device structures produced according to the invention may function as replacements, or subcomponents, for known electrical elements such as capacitors, inductors, transistors, diodes, logic gates, circuits, actuators, sensing elements, pumps, and so on.
The development efforts within the electronics industry continues to head in the direction of developing extremely tiny, yet reliable, electronic components. However, conventional means to do so will soon, or has, hit the point at which even a very small reduction in size can only be achieved at great cost. Unlike the conventional limited component/device fab systems currently in use, the apparatus and associated method of the invention, as well as the conductive devices produced thereby, utilize an innovative approach. None of the currently-available microcomponent/microcircuit fabrication systems disperse a carrier-fluid with nano-elements directed at a target charge-receptive area(s) that carries a charge to attract elongated conductive element(s), for example carbon nanotubes or nanorods, according to a final desired useful device structure.
Certain advantages of providing the flexible new apparatus, conductive devices, and associated new method, as described herein, include without limitation:
(a) Nano-sized device fabrication cost reduction.
(b) Device, apparatus, and process design flexibility and versatility.
(c) Process simplification.
(d) Device design flexibility.
Briefly described, once again, the invention includes an apparatus for dispersing a first plurality of conductive elongated nano-elements distributed within a carrier-fluid to assemble a conductive device. The apparatus includes: a nozzle through which the elongated nano-elements are directed such that the nano-elements pass through an electromagnetic field for imparting a preselected charge thereto, and toward at least a first charge-receptive area of a support surface. The charge-receptive area has a charge such that it attracts a first end-portion of one of the nano-elements. The assembled conductive device comprises the first charge-receptive area to which at least the nano-element has attached. The nano-elements include those nano-sized structures having an aspect ratio on the order of 100 to 10,000, whether hollow, capsule-like, or filled-in. The carrier-fluid may be in liquid form (nano-elements dispersed within at least one droplet), preferably substantially evaporating once the nano-element has attached; or in gas form, preferably dispersed as a high pressure fluid or the nozzle and charge-receptive area being under vacuum (decrease defects due to turbulent flow).
Also characterized is a method of assembling a conductive device. Steps include: applying a first charge to the first charge-receptive area to attract a first end-portion of the one nano-element; and dispersing from a nozzle, the plurality of elongated nano-elements distributed within a carrier-fluid initially contained in a reservoir, such that the nano-elements pass through an electromagnetic field for imparting a preselected charge thereto and toward the first charge-receptive area. The step of applying a respective charge to a charge-receptive area can include: applying a first voltage potential to the area such that the charge is retained at least until the target nano-element so attaches; or suitably depositing charged ions to each respective area; and so on. A second plurality of nano-elements may be passed through a second electromagnetic (EM) field, generated for example by a suitably variable charging electrode, for imparting a second preselected charge thereto. In this manner, xe2x80x98wavesxe2x80x99 of groups of nano-elements may be passed through various different EM fields to impart respective preselected charges to the groups of nano-elements; thus, providing directional control allowing different charge-receptive areas of various charges (whether the charge(s) are given to the areas initially, or thereafter but prior to the dispersion of the particular wave/group of targeted nano-elements) to attract selected waves of nano-elements.
A conductive device assembled according to the invention can further comprise a second charge-receptive area located a distance, d, from the first charge-receptive area; as well as third and fourth such areas (collectively forming one or more patterns of areas to which nano-elements may be attracted and thereafter affixed, more-permanently, such as by application of an aerosol adhesive, one or more spot welds employing suitable solder, one or more spot application of thermal energy, and so on). The charge-receptive areas may comprise deposits of ions, localized conductive areas such as a via or pad-like structure to which a variable voltage potential may be applied, and other suitable means of applying a respective localized charge to the support surface/structure. All or a select few of the charge-receptive areas may be generally electrically-insulated from one another according to a final device structure. For example, each of the charge-receptive areas may comprise a protuberance electrically insulated from one another to retain a respective independently controllable charge. To provide further design flexibility, the second charge-receptive area may be charged to initially repel the first end-portion of the nano-element and to attract a first end-portion of a second nano-element.
In the event a second charge-receptive area is located a distance, d1, from the first charge-receptive area, where d1 is less than an average length, l, of the nano-element and a first end-portion is attached to the first area, attaching the other end-portion of the nano-element can be done such that a bridge shorting the two charge-receptive areas is formed. To discourage such nano-element bridge shorts, for example, between the first and a third charge-receptive area (separated a distance, d2) and between a second and the third charge-receptive area (separated a distance, d3), d2 and d3 can be made greater than length, l. Or in this case, more than one nano-element may be employed in chain-fashion to short the first and third areas.
Further distinguishing specific features of the apparatus and method include: dispersing a second plurality of conductive elongated nano-elements within the carrier-fluid such that the second plurality passes through a second electromagnetic field for imparting a second preselected charge thereto; a variable charging electrode for generating a selected initial EM field and controlled to generate subsequent electromagnetic fields through which later waves/groups of conductive elongated nano-elements may be passed for imparting associated respective preselected charges thereto; also a pair of deflection plates between which the nano-elements pass after passing through a respective electromagnetic field but prior to attachment to a respective charge-receptive area.
Additional, further distinguishing associated features of the apparatus for assembling a conductive device and method can be readily appreciated accordingly.